This invention relates to electrophoretic display panels in general and more particularly to a method of eliminating gas bubbles during operation of such an electrophoretic display.
The electrophoretic effect is well known and the prior art is replete with a number of patents and articles which describe the effect. It is sufficient to state that researchers have been working with the electrophoretic effect for many years. Essentially, the electrophoretic effect operates on the principle that certain pigment particles will become electrically charged and because of being electrically charged these particles can migrate from a like charged surface to an opposite charged surface.
Many of the prior art references describe various particle materials which when suspended in solution exhibit the electrophoretic effect. The effect is associated with a working fluid or solution as a dispersion including pigment particles. These particles may be fabricated from opaque dielectric materials such as Titania and other pigments which particles are suspended in a colored non-conducting suspension medium to form a dispersion. The particles are distributed uniformly throughout the suspension medium with the suspension medium being of a contrasting color to the particle color. When the composite material is subjected to an electric field the particles are caused to move electrophoretically in the direction of either the cathode or anode. These particles are deposited upon a respective structure to cause that structure to assume the color of the particle. Hence, as one can see, by selectively moving the particles one can produce images based on the migration and orientation of the particles with respect to a charged surface. The effect is well known and used for displays.
See for example U.S. Pat. No. 4,732,840 issued on Mar. 27, 1988 to DiSanto et al. and entitled "Electrophoretic Display Panels and Associated Methods" and assigned to Copytele, Inc., the assignee herein. This patent basically describes the structure of an electrophoretic display including the components of the display. Also see U.S. Pat. No. 4,742,345 issued on May 3, 1988 to DiSanto et al. and entitled "Electrophoretic Display Panel Apparatus and Methods Therefor" and assigned to the assignee herein. Also see U.S. Pat. No. 4,746,917 issued on May 24, 1988 to DiSanto et al. and entitled "Method and Apparatus for Operating an Electrophoretic Display Between a Display and a Non-Display Mode". Also see U.S. Pat. No. 4,772,820 issued on Sept. 20, 1988 to DiSanto et al. and entitled "Monolithic Flat Panel Display Apparatus" and assigned to the assignee herein.
It has been discovered that when such electrophoretic displays are operated gas bubbles which are inherently present in the dispersion due to the fabrication processes tend to gather together and grow in size. This bubble growth causes voids in the display and the bubbles as formed can actually be seen. The bubbles, when the display is originally produced, are extremely small in size and as the display is operated, by placing suitable biasing potentials on the display electrodes, the bubble size grows. This is further enhanced due to the turbulence created by the transported pigment particles as they do not move in a linear manner or with constant velocity. These rather large bubbles create flaws in the viewing surface of the display and oftentimes can render the display unusable.
It is believed that the cause of the bubbles are due to gases such as dissolved air which is always present when the dispersion is interposed between the suitable display electrodes. The particle suspension fluids are non polar and hence they tend to absorb more air. It is desirable to avoid any bubbles of gas during the filling and sealing of the cell or display and to maintain the number of bubbles as small as possible. In any event, as indicated above, the dispersions contain solvents with dissolved gases and high surface area pigment particles with absorbed gases. These all tend to generate bubbles of gas when the cell operates. As operation of the cell increases the bubble size increases whereby the small minute bubbles join together to form layer bubbles which, as indicated above, serve to render portions of the viewing surface of the display unusable.
For examples of suitable fluids which operate in electrophoretic displays, reference is also made to U.S. Pat. No. 4,093,534 which issued on June 6, 1978 and entitled "Working Fluids for Electrophoretic Image Display Devices" by C. F. Carter et al. This patent provides a working fluid which includes a dispersion of finely divided particles of an opaque dielectric material suspended in a suspension medium. The particles are transportable within a suspension medium under the influence of an electric field. This patent describes various structures which produce a display operating according to the electrophoretic phenomenon.
U.S. Pat. No. 4,285,801 entitled "Electrophoretic Display Composition" issued on Aug. 25, 1981 to A. Chiang and describes a suspension for an electrophoretic display where the particles are coated with a highly fluorinated polymeric material which acts as the dispersant. Essentially, as indicated, the particles are chosen to optically contrast with the medium. Accordingly, the optical contrast may result from using light reflecting versus light absorbing materials or materials of different colors as above indicated.
See also U.S. Pat. No. 4,272,596 issued on June 9, 1981 to J. R. Harbour et al. entitled "Electrophoretic Display Device" and assigned to the Xerox Corporation. This patent describes an electrophoretic display which provides high contrast images in a stable suspending fluid. See also U.S. Pat. No. 4,077,936 entitled "Liquid Suspending Medium for Electrophoretic Image Display/and or Recording Device" issued on May 7, 1978 to S. Tanaka et al. and assigned to Matsushita Electric Industrial Co. This patent describes a liquid suspending medium for an electrophoretic image display which contains an oligomer of 1,1,2-trifluoro-2-chloroethylene. The use of this oligomer purportedly facilitates gravity adjustment between a liquid suspending medium and electrophoretic particles and results in a liquid suspending medium suitable for an electrophoretic image display.
As one can understand, the solvents employed in such displays are associated with dissolved gases which dissolved gases are mainly air and tend to accumulate to produce large bubbles during display operation while particle motion also causes bubble formation. The larger bubbles interfere with the visual aspects of the display. It is immediately noted that in any of the above devices or displays such bubble generation occurs during prolonged display or cell operation.
It is therefore an object of the present invention to provide a method for eliminating gas bubbles in an electrophoretic display.
It is a further object of the present invention to utilize an ultrasonic probe which is placed into the dispersion medium to cause bubbles to rapidly be removed from the display without damaging the dispersion.
It is therefore another object of the present invention to provide an improved electrophoretic display having minimum bubble generation during operation.